Under a dome

ByMargaret D'EvelynJune 26, 1987

On a beautiful day, do you ever think that the sky looks like a great blue bowl turned over you? Do you feel like reaching up your arms in a circle as if to touch the sky with your fingertips? You're not alone! For thousands of years people have thought of the sky as a big blue bowl. That inspired architects to make buildings to match.

One of the largest domes and the ``father'' of many other domes, the Pantheon, has been around for more than 18 hundred years, first as a Roman temple, then as a Christian church.

The dome of the Pantheon looks like a hill resting on a very broad oatmeal box - with a columned porch (portico) outside. Inside, a huge round room opens up like a 150-foot hollow ball, sliced in half, sitting on those ``oatmeal box'' walls. Bright sun shines in through the round, 30-foot-wide hole (oculus) in the top!

Let's imagine we're living in Rome AD 118. We'll help to build that magnificent dome! First, let's mix up a batch of concrete. We'll add two buckets of sand to one of lime, blending in rocks and water.

To construct the circular wall, let's pour our concrete into a space between special low brick walls, built by our friends. While we work, our first batch will dry in the sun, and our friends will build the framing walls higher.

Five thousand tons of concrete, please, for the dome! This time, as clever Roman builders, let's add bits of volcanic rock, not heavy rocks, to our mixture. This light pumice makes a strong, airier concrete to stretch across the huge space inside - first on wood scaffolding supports, then, when it's dry, all on its own!

Now that you're an expert on building concrete domes, maybe you'd like to meet some others. Maybe there's one right in your pocket! Our third President, Thomas Jefferson, designed a small dome for his hilltop home, Monticello, in Virginia. Have you seen its portrait on the back of a nickel?

If you don't have a nickel right now, perhaps you have a letter with a 22-cent stamp? Can you find the Capitol dome, built during the Civil War and looking like a finely cut stone dome? Good! Would you like to know a secret? That dome is made of cast iron painted white (a faster and cheaper way to build, discovered just at that time).

Do you see the circular wall (the drum) that supports the dome on top of the congressional building? High columns (the peristyle) wrap around the drum, making spaces for windows. A smaller peristyle (the lantern) caps the whole mountainlike pile!

Now you're ready to go out and enjoy all the big and little domes, the ``children'' of that great concrete dome that first echoed the glories of the ``vault'' of the sky long, long ago.